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1 Damper 2 Furnaces?

My boss ask if two furnaces can control one damper independantly. I have not seen the job but was told there is one fresh air inlet that is piped into two seperate furnaces via the return duct. This air damper is to open, only when at least one of the two furnaces are running, does not matter which, and to be open when both are running. The furnaces are not twined, each have a seperate T-Stat and control different rooms. How can I control this damper with two furnaces, but not to back-feed to the furnace that is off?

Thanks, Keith

I'm an HVAC/R service Tech! Just have not got enough post to be pro yet.

Use a current switch on the blower common wire for each furnace and wire the switches in parallel using 24 volts from somewhere. Just make sure the transformer has enough va to stroke the damper actuator..

When one of the two blowers, or both are on, it closes the current switch(s) and commands the damper to open..

nice and easy.

Im sure more people will chime in with different solutions, theres more than one way to skin a cat.

power the OA motor drive direct then add one relay per boiler 24V coil N.O. contacts in stacked series with the hot wire motor voltage either or will drive it on a heating call..if the boilers are commercial size you might have dry N.O. contacts within the control panel there

I could use two current sensing relays on the G wire of each furnace. When ever the sensor picks up current the relay would close and power the damper. This would allow for a seperate transformer to power the damper correct? I know there are avalible items from electronic suppliers, but has anyone seen one at the HVAC supplier? Would one of these sensor pick up the slight current going through the G wire?

I've only seen two furnaces on the same duct system if they were twinned. If they aren't twinned, wouldn't there need to be some sort of barometric damper, just in case only one of the furnaces are running? If only one was running, couldn't the supply air blow backwards through the furnace that isn't running? I could be wrong, but I could imagine that type of scenario happening, if there wasn't a damper to stop the air from going back through the furnace that's off.

That's it!!! I can't believe I did not think of that, I feel like a dumb

I was told the damper opens and lets fresh combustion air for each furnace, still have never seen the job.

Thanks

If you are using it to bring in fresh air when the heat is on you need to wire it in to w-c not g-c it will only open when you have the fan turned on or the ac is on. I would think you need it for heat. thinking it for freash air to the furnace room.
you could put it on a curnant sensing relay on the blower motor to a relay for the damper.